1. Field of the Invention
The present invention relates to a CAPACITIVE SENSOR such as an Si (Silicon) microphone, etc., and a method for manufacturing the same.
2. Description of Related Art
In recent years, in place of ECM (Electret Condenser Microphone), mobile phones have started to be equipped with Si microphones manufactured by MEMS (Micro Electro Mechanical Systems) technology, therefore Si microphones have been dramatically attracted attention.
For ECMs, it is impossible to fix on a wiring board by soldering. If the ECM is at a high temperature for reflow, electric charge leaks from a macromolecular film that is a thin film to sense sound pressure, so that the ECM will stop functioning as a microphone. Therefore, the ECM is fixed on a wiring board by using an exclusive component. As opposed to this, since an Si microphone can be reflowed at a high temperature that exceeds 250° C., it is possible that the Si microphone is mounted on a wiring board by an automatic mounter as with other semiconductor components. Therefore, it is possible to lower the mounting costs by using an Si microphone.
FIG. 4 is a schematic sectional view showing the structure of a prior art Si microphone.
An Si microphone 101 includes a silicon substrate 102. A through-hole 103 is formed, whose section is trapezoidal and inner diameter is gradually widened toward the rear side thereof at the center of the silicon substrate 102. A thin film 104 formed of polysilicon is disposed on the surface of the silicon substrate 102 so as to block the through-hole 103. Also, a back plate 105 formed of polysilicon is disposed on the thin film 104 so as to be opposed to the thin film 104. A number of holes 106 are formed through the back plate 105. The back plate 105 is supported at the peripheral edge portion thereof by the supporting frame 107, and a cavity 108 having minute spacing is formed between the thin film 104 and the back plate 105. Accordingly, the thin film 104 and the back plate 105 are opposed to each other with the cavity 108 sandwiched therebetween and form a capacitor.
When sound pressure (sound waves) is inputted in the Si microphone 101, the thin film 104 is oscillated by the sound pressure, whereby electric signals corresponding to changes in the electrostatic capacitance of the capacitor, which are generated by oscillation of the thin film 104, are outputted.
However, in a manufacturing process of traditional Si microphones 101, since the thin film 104 is formed on the surface of the silicon substrate 102, it is necessary that an etching solution is supplied onto the silicon substrate 102 from the rear side thereof in order to form the through-hole 103 on the silicon substrate 102. On the other hand, the cavity 108 between the thin film 104 and the back plate 105 is formed by etching a silicon oxide thin film after accumulating silicon oxide on the thin film 104 and forming the back plate 105 on the thin film formed of the silicon oxide. However, for the etching, it is necessary for an etching solution to be supplied through the hole 106 formed on the back plate 105. For this reason, it is impossible that a step for forming the through-hole 103 and a step for forming the cavity 108 are carried out in the same process.